Shaped activated carbon articles are used in the automobile industry, in particular in tank venting systems, for the reduction of evaporative emissions from a motor fuel supply system. Hydrocarbons emitted by a motor fuel reservoir as a result of an increase in pressure, particularly when the motor vehicles are not in motion and are exposed, for example, to direct solar radiation in the summer months, are retained by such tank venting systems to prevent such emissions from passing into the environment.
The tank venting systems or tank venting filters can consist of single-chamber or multi-chamber systems having activated carbon packings. Twin-chamber systems are disclosed in U.S. Pat. No. 5,957,114 or U.S. Pat. No. 6,503,301.
DE 199 52 092 C1 discloses an activated carbon filter which comprises a filter section containing activated carbon and a filter layer of high capacity adsorbent containing a material comprising zeolite and/or silica gel and/or alumina and/or divinylbenzenestyrene.
WO 01/62367 discloses a method for the adsorption of hydrocarbon vapors from motor fuel gas mixtures, in which the gas mixture is initially passed through a first adsorption system and then through a second adsorption system, and in which the adsorption rate of the second adsorption system is higher than the adsorption rate of the first adsorption system. The first and second adsorption systems can both consist of activated carbon, the surface area to volume ratio of which differs in the two systems.
The aforementioned filter systems all use activated carbon packings, which suffers from the drawback that they all show a very high pressure drop.
In order to achieve good regenerability of activated carbon packings, the particle size of the activated carbon used must be as small as possible. The use of activated carbon of such minimum size leads adversely to an even greater pressure drop in the aforementioned systems.
If a tank venting system or a tank venting filter is to show a low pressure drop combined with good regenerability, use must be made of monolithic structures containing an adsorbent.
U.S. Pat. No. 4,386,947 discloses a device for the adsorption of motor fuel vapors, in which first, second and third monolithic structures comprising activated carbon are laminated together such that the passages in the monolithic structures form a zigzag path, through which the gas or vapor is passed. This arrangement is too complicated for normal manufacturing technology and too cost-intensive in regard of the fact that the tank venting systems are mass products.
U.S. Pat. No. 6,540,815 B1 discloses a method for the reduction of motor fuel vapors in automobiles, in which the vapors are passed initially through an activated carbon packing and subsequently through an activated carbon-containing shaped ceramic article having a honeycomb structure. The method disclosed in U.S. Pat. No. 6,540,815 B1 suffers from the disadvantage that, on the one hand, an activated carbon packing must be used, which, as explained above, produces a high pressure drop, and, on the other hand, the activated carbon-containing ceramic honeycomb filter contains not more than approximately 35% by weight of activated carbon on account of the content of ceramics material and consequently has a restricted adsorptive capacity.
A further disadvantage arising when use is made of activated carbon packings in a motor vehicle is that when the motor vehicle is in motion the activated carbon packing is subjected to vibrations causing the activated carbon particles to rub against one another. The rubbing of the activated carbon particles against one another produces abrasion and leads to pulverization of the activated carbon, whereby the adsorptive capacity of the activated carbon packing is impaired.
It is an object of the invention to provide an adsorbent, preferably for the adsorption of hydrocarbon vapors, in a form which enables a good adsorptive capacity to be combined with a low pressure drop.